PHF Science research shows vaccination protects against virus spread between family members living together.
Being vaccinated against Covid-19 protects the health of your family by preventing spread of the virus from infected household members to those uninfected, found a New Zealand study published in Vaccine:X.
The study assessed the household ‘secondary attack rate’ of Covid-19, which is the proportion of family members who become infected when living with a family member who is infected (known as the ‘index case’). Vaccinated family members who received two or three doses were less likely to get infected, and vaccinated infected people were less likely to pass it on to other family members. In households where everyone was unvaccinated or received only one vaccine dose, 88 per cent of household members caught the virus, 1.6 times more than in households where all members were vaccinated at least twice.
Understanding how Covid-19 spreads within a household helps to inform future modelling for predicting viral transmission, planning mitigation scenarios, and preparing for future pandemics.
“The data from this paper showed that vaccination significantly reduced household transmission of the Covid-19 Omicron variant,” says the paper’s first author Dr Nayyereh Aminisani, lead epidemiologist of the SHIVERS (Southern Hemisphere Influenza and Vaccine Effectiveness Research and Surveillance) study. “This suggests the vaccine offers protection for individuals, families and the wider community.”
Vaccine studies typically measure reduction in hospitalisation or GP consultations. However, because most cases of Covid-19 were mild and didn’t require medical support, the study has filled an important knowledge gap about the effect of vaccination on mild illness, particularly in a household setting, says Professor Sue Huang, principal investigator of SHIVERS and director of the WHO National Influenza Centre at PHF Science.
Understanding the scale and spread of mild cases can help support the public health responses at the end of a pandemic, when a disease is transitioning to become endemic alongside other respiratory infections.
“If you want to reduce the illness personally and for your family, this scientific evidence shows it is very important to have the vaccine for your family,” says Huang.
The research was based on information gathered through PHF Science’s SHIVERS research platform. Through SHIVERS, 75 families with 306 household members agreed to be part of the Covid-19 study from February to October 2022. (The highly transmissible Omicron variant of SARS-CoV-2 arrived in January 2022, prompting New Zealand’s transition to a mitigation strategy in February 2022.)
When a household member tested positive for the virus, all members took swabs every three days, whether they were sick or not. “The study data is available because of the dedicated participation of the SHIVERS household members,” says Huang. “Household transmission is often a critical driver for community-wide outbreak, and this type of platform is excellent for pandemic preparedness and response for New Zealand.”
PHF Science Chief Executive Sir Ashley Bloomfield, New Zealand’s Director-General of Health at the time, says this study’s finding is significant because it demonstrates the usefulness of the vaccine in reducing community transmission.
“A common view was that vaccines were not very effective in preventing transmission of the newer variants, especially Omicron. The particular significance of this finding is vaccination does reduce onward transmission of the virus, emphasising the value of vaccination for those working with high-risk people.”
As well as supporting the usefulness of the vaccine in helping prevent illness, the findings suggest sharing bathrooms, hand towels, and hugging may lead to household transmission of the Covid-19 virus. This aligns with previous studies that recommend preventive measures such as handwashing, surface cleaning, physical distancing, and wearing masks at home.
The research also looked at the demographics of transmission. In 60 per cent of cases, the first people in the family to test positive for Covid-19 were under 18, and teenagers had the highest incidence of infection. People of European ethnicity had the highest incidence rate. The risk of infection was higher among families living in lower socioeconomic conditions with more individuals in the family home, although the sample size for these families was low.
PHF Science’s laboratory and genomic epidemiology capabilities enabled the genome of the viruses to be sequenced to identify sub-variants of Omicron. For two households in the study, published in NPJ Viruses in 2024, this revealed different variants within the home, indicating more than one family member had contracted Covid-19 outside the home at the same time.
“Knowledge is powerful,” says Lauren Jelley, PHF Science virologist and SHIVERS team member. “The more information we have about viruses that are circulating, previous exposure and vaccination history, the better we are able to plan for what happens in the future.”
Sources:
Aminisani, N., Huang, Q. S., Jelley, L., Wood, T., Jennings, T., Currin, D., Seeds, R., Marull, M., Kvalsvig, A., Baker, M. G., Nghiem, N., Thomas, P. G., & Webby, R. J. (2025). Household transmission of the SARS-CoV-2 omicron variant in New Zealand, 2022. Vaccine X, 24, 100638. https://doi.org/10.1016/j.jvacx.2025.100638
Jelley, L., Nayyereh Aminisani, O’Neill, M., Jennings, T., Douglas, J., Srushti Utekar, Johnston, H., Welch, D., Hadfield, J., Turner, N., Dowell, T., Nesdale, A., Dobinson, H. C., Campbell-Stokes, P., Balm, M., Grant, C. C., Daniells, K., McIntyre, P., Trenholme, A., & Byrnes, C. (2024). Tracing household transmission of SARS-CoV-2 in New Zealand using genomics. Npj Viruses, 2(1). https://doi.org/10.1038/s44298-024-00032-6